1. Field of the Invention
This invention relates to a pneumatic radial tire, and more particularly to a high-performance pneumatic radial tire capable of considerably improving straight running stability and steering stability while maintaining other properties at a high level and suitable for use in vehicles such as passenger cars.
2. Description of Related Art
In order to enhance the wet skid resistance and resistance to hydroplaning required in a high-performance tire, when a tread pattern formed in a tread rubber is a pattern including block groups defined by at least two main straight grooves extending along a circumference of a ground contact region and lateral sub-grooves arranged therebetween or a block pattern formed over substantially a full ground contact region, a level of noise called as a pattern noise is apt to be increased as compared with that in a rib pattern. Therefore, in order to reduce such a noise level, it is required to adopt a pitch variation wherein pitch lengths arranged in the pattern along the circumference of the ground contact region are changed at a given ratio.
When the pitch variation is applied to rows of many blocks along the circumference of the ground contact region, there is a first case where inclination angles of block wall faces along the straight main groove are the same in the same block row, or even if the inclination angle is changed in one block in view of the pattern design, the change of the inclination angle is the same in all blocks of the same block row irrespective of various pitches.
There is a second case where inclination angles of the block wall faces along the lateral sub-groove arranged for defining the block and having a given inclination angle with respect to a direction of tire rotating axis (hereinafter abbreviated as axial direction) in the same block row are (1) the same in all blocks for facilitating the manufacture of a vulcanization mold or (2) are made larger in a long pitch than those in a short pitch.
In order to ensure a good drainage property during running on a wet road surface, it is general known to incline the lateral sub-grooves in the same direction with respect to the axial direction in the same block row. In this case, it is known that the inclination angle of the lateral sub-groove with respect to the axial direction is made larger in the block of long pitch than in the block of short pitch or is the same irrespective of various pitches as a third case.
In the tire having a combination of the first and second cases with the third case wherein the inclination angles of the lateral sub-grooves differ in blocks having long and short pitches, values obtained by dividing axial rigidities of long-pitch and short-pitch blocks by their pitch lengths or axial rigidities of long-pitch and short-pitch blocks per unit length along the circumference of their ground contact region become smaller in the long-pitch block having a larger degree of acute angle and larger in the short-pitch block having a smaller degree of acute angle.
In the pattern having block rows of many pitch-varied blocks, larger rigidity zones and smaller rigidity zones are existent together in a tread rubber portion in the ground contact region of the tire rotating under loading in the axial direction. Because, it is effective to arrange blocks having the same pitch only by certain number in order to more enhance the effect through the pitch variation and hence the block row is divided into certain number of long-pitch block groups and certain number of short-pitch block groups. Therefore, the tread portion has long-pitch and short-pitch block rigidity zones in the axial direction along the circumference of the ground contact region. The result is that the block row having uniform rigidity zones is not obtained.
Such a presence of long-pitch and short-pitch block rigidity zones along the circumference brings about the change of cornering force quantity produced every one rotation of the tire or the change of cornering power. Such a change does not count for so much in the low speed to middle speed running of the vehicle, but causes swaying of the vehicle from side to side in the high-speed straight running or a problem of degrading the straight running stability and the steering stability in the cornering at a relatively high speed. In case of high-performance radial tires to be lately and increasingly mounted onto a high-performance vehicle, it is particularly demanded to highly develop the above both performances, so that the above problem becomes serious.
In the tire having a combination of the first and second cases with the third case wherein the inclination angles of the sub-grooves in all blocks are the same irrespective of the various pitches, there is a risk of simultaneously contacting the sub-grooves the ground in the running under loading, which brings about the occurrence of tire noise through impact sound or the like. Therefore, it is a general means to arrange grooves extending in the lateral direction in the circumferential direction while changing the inclination angle.
It is, therefore, an object of the invention to provide a pneumatic radial tire provided with a tread pattern having a row of many blocks formed under pitch variation contributing to lower noise which can improve both straight running stability and steering stability while maintaining a necessary low noise level and further holding other performances such a ride comfortability and the like at a high level.
According to a first aspect of the invention, there is the provision of a pneumatic radial tire comprising a radial carcass of at least one ply toroidally extending between a pair of bead cores each embedded in a bead portion, a belt reinforcing a tread portion on an outer periphery of the carcass, and a row of many blocks having different lengths along a circumference of a ground contact region under pitch variation defined in a tread rubber of the tread portion by at least two straight main grooves extending along the circumference of the ground contact region and a plurality of lateral sub-grooves extending between the mutual main grooves at an inclination angle of less than 90xc2x0 with respect to the circumference of the ground contact region, in which the block row satisfies a relationship of xcfx86L greater than xcfx86S as to an inclination angle (xcfx86) of an edge of the block along the sub-groove with respect to a straight line passing through a top of an acute angle portion of the block and perpendicular to an equatorial plane of the tire wherein xcfx86L is an inclination angle of a long-pitch block and xcfx86S is an inclination angle of a short-pitch block, and a relationship of xcex1L greater than xcex1S as to an inclination angle (xcex1) of each block wall face along both the straight main grooves with respect to a length of each block surface along the circumference of the ground contact region wherein xcex1L is an inclination angle of a block wall face having a longer surface length and xcex1S is an inclination angle of a block wall face having a shorter surface length.
According to a second aspect of the invention, there is the provision of a pneumatic radial tire comprising a radial carcass of at least one ply toroidally extending between a pair of bead cores each embedded in a bead portion, a belt reinforcing a tread portion on an outer periphery of the carcass, and a row of many blocks having different lengths along a circumference of a ground contact region under pitch variation defined in a tread rubber of the tread portion by at least two straight main grooves extending along the circumference of the ground contact region and a plurality of lateral sub-grooves extending between the mutual main grooves at an inclination angle of less than 90xc2x0 with respect to the circumference of the ground contact region, in which the block row satisfies a relationship that as to an inclination angle (xcex2) of each block wall face along the sub-groove with respect to a length of each block surface along the circumference of the ground contact region, an inclination angle (xcex2S) of a block wall face having a shorter surface length is made larger than an inclination angle (xcex2L) of a block wall face having a longer surface length.
Of course, the tire can be manufactured by combining the first aspect with the second aspect. In each of the first and second aspects, the pitch variation is a random arrangement of two or more pitch lengths, desirably three or more pitch lengths.
In a preferable embodiment of the invention, a ratio of a full volume of a block to a full volume of a groove existing in each pitch defined by different pitch lengths based on the pitch variation is made even over a full is circumference of the ground contact region. In this case, the full volume of the groove existing in each pitch is a groove volume existing in a space defined by four faces dividing each of groove widths of the two main grooves and the two sub-grooves for the formation of a certain block into two parts and a ground contact face and its extension face, while the full volume of the block is a volume of a block occupied in a solid defined by the above faces and a phantom face (usually a curved face) smoothly connecting groove bottoms of the two main grooves to each other. Also, the even value of the ratio means a value that a difference between the maximum value and minimum value in the ratio of the full volume of the block to the full volume of the groove existing in each pitch is not more than 0. 1.
In another preferable embodiment of the invention, the inclination angle of the block wall face directing from the surface of the block toward the bottom of the main groove is gradually decreased over a region ranging from an acute end to an obtuse end in a surface form of each block, which enhances the rigidity of each block in the axial direction and largely contributes to improve both the straight running stability and the steering stability.